Drug interactions
carbamazepine + phenytoin
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Overview
Bidirectional PK interaction: each induces the other’s metabolism. Phenytoin lowers carbamazepine; carbamazepine may lower or occasionally raise phenytoin. Net effect can be unpredictable; risk of toxicity or loss of seizure control.
Mechanism of action
Both are strong enzyme inducers (CYP3A4/2C9/2C19, UGT, P-gp). Phenytoin induces carbamazepine metabolism → ↓carbamazepine. Carbamazepine induces phenytoin metabolism and may inhibit CYP2C19 → variable phenytoin levels.
Clinical management
Monitor serum carbamazepine and phenytoin concentrations and clinical response after initiation/changes; adjust doses accordingly. Recheck levels within 1–2 weeks (earlier for toxicity). Avoid abrupt changes; consider alternatives if levels unstable.
Drug reference
References
- Tegretol (carbamazepine) Prescribing Information — FDA Label
- Dilantin (phenytoin) Prescribing Information — FDA Label
- Carbamazepine–Phenytoin: Drug Interactions — Lexicomp
Frequently asked questions
Can I take carbamazepine with phenytoin?
The interaction is rated: Moderate. Bidirectional PK interaction: each induces the other’s metabolism. Phenytoin lowers carbamazepine; carbamazepine may lower or occasionally raise phenytoin. Net effect can be unpredictable; risk of toxicity or loss of seizure control.
How serious is the interaction between carbamazepine and phenytoin?
The interaction is rated: Moderate. Evidence level: High.
How is the interaction between carbamazepine and phenytoin managed?
Monitor serum carbamazepine and phenytoin concentrations and clinical response after initiation/changes; adjust doses accordingly. Recheck levels within 1–2 weeks (earlier for toxicity). Avoid abrupt changes; consider alternatives if levels unstable.
Why do carbamazepine and phenytoin interact?
Both are strong enzyme inducers (CYP3A4/2C9/2C19, UGT, P-gp). Phenytoin induces carbamazepine metabolism → ↓carbamazepine. Carbamazepine induces phenytoin metabolism and may inhibit CYP2C19 → variable phenytoin levels.
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